1. Field of the Invention
The present invention relates to communication systems, and in particular, to the filtering of radio frequency communication signals.
2. Background Art
Wireless communication systems enable the transfer of information over a distance without the use of electrical conductors or wires. Such wireless communication systems typically transmit radio frequency (RF) communication signals that are modulated with information. Varieties of communication standards exist that define attributes of communication signals for use in different applications. For example, a communication signal according to a particular standard may have a defined carrier frequency, one or more defined modulation schemes, a defined error correction scheme, and/or further defined attributes.
A wireless communication system may include one or more transmitters and receivers. A transmitter is used to transmit a RF communication signal, while a receiver is used to receive the transmitted RF communication signal. A transmitted communication signal has a bandwidth, which is a difference between upper and lower cutoff frequencies for the signal. For example, FIG. 1 shows a graph 100 of frequency versus amplitude for a RF communication signal 102. RF communication signal 102 is illustrated in FIG. 1 as having a center (e.g., carrier) frequency of f1 and a bandwidth BW. A signal near RF communication signal 102 in frequency may be received by a receiver attempting to receive communication signal, and thus may interfere with RF communication signal 102. Such a signal may be referred to as a “blocker signal.” For example, FIG. 1 shows a blocker signal 104, having a frequency of f2 that is near frequency f1 of RF communication signal 102.
FIG. 2 shows a conventional receiver 200 that may be used to receive RF communication signal 102. As shown in FIG. 2, receiver 200 includes an antenna 202, a SAW (surface acoustic wave) filter 204, an LNA (low noise amplifier) 206, and a down-converter 208. As shown in FIG. 2, antenna 202 receives RF communication signal 102, which is desired, but may also undesirably receive blocker signal 104 with RF communication signal 102. SAW filter 204 is coupled to antenna 202, and receives RF communication signal 102 and blocker signal 104 from antenna 202. SAW filter 204 has a pass band configured to pass RF communication signal 102 and a stop band configured to filter out blocker signal 104. SAW filter 204 generates a filtered communication signal 212, which is received by LNA 206. LNA 206 amplifies filtered communication signal 212, generating an amplified filtered communication signal 214. Down-converter 208 receives amplified filtered communication signal 214, and generates a down-converted output signal 216, which is typically a baseband information signal.
In many receivers, such as receiver 200, a stringent blocking requirement must be met. For instance, in the GSM (Global System for Mobile communications) standard, RF communication signal 102 may be received at 3 dB above a sensitivity level, and may be accompanied by a 0 dBm blocker signal 104 that may be as close as 80 MHz to the edge of the PCS (Personal Communications Service) band of signal 102. In this case, because the desired signal is relatively weak, the gain of LNA 206 must be relatively high, and thus blocker signal 104 must be well filtered by SAW 204. Due to the modest Q of on-chip inductors, it is not practical to integrate a SAW filter 204 having such filter sharpness in the same integrated circuit chip as LNA 206 and/or the rest of receiver 200. For these reasons, SAW filter 204 is external to the integrated circuit chip from LNA 206.
Disadvantages are caused by having SAW filter 204 off chip from other portions of receiver 200. First, cost is increased due to an increase in a number of components, especially in multi-mode, multi-band applications. Second, the insertion loss of SAW filter 204, which is typically as high as 2-3 dB, degrades the sensitivity of receiver 200. Third, there is less flexibility in sharing LNA 206 in multi-band applications, and particularly in software-defined radios.
Thus, what is desired are ways of filtering RF communications signals in a manner that reduces costs, avoids problems with insertion loss, and provides flexibility in multi-band applications, as an alternative to the use of SAW filters.